Abrasive disks, or sanding disks are widely used on portable electric drills and (at a more professional level) on hand-held angle grinders. When used on these machines the disk is held by its centre against a backing pad and is rotated at generally a high speed while pressed in front of a backing plate against the work. The abrasive surface wears down the surface of the work by, in effect, a cutting action. Angle-grinder mounted sanding disks are commonly used (for example) in automotive panel beating, where body filler is to be sanded back to conform to the original contours of a remodelled car part. It is said that millions of sanding disks suitable for use with angle grinders are sold each year. There are some problems related to the use of sanding disks, such as:
(a) The relatively rigid backing disks commonly used with angle grinder sanding disks force the sanding disks into an unsatisfactory mode of operation when the angle grinder is tilted towards the work during use--such as that primarily the edge engages with the work resulting in local, intense action rather than an even, gradual action over a wider area. There is a tendency for the work surface to develop an unsatisfactory scalloped surface which requires hand sanding block treatment. The disks cannot be used for finely controlled work such as preparation of surfaces in a state ready for painting. PA1 (b) Sometimes the material being abraded tends to melt at the high cutting speeds involved, and if this happens it is particularly likely to clog the sanding disk in a quick and effective manner so that the disk has to be discarded. Melting may also lead to the tool biting in and as a result the surface of the work may be inadvertently destroyed. Heating also adversely affects the life of the sanding disk. PA1 (c) The operator cannot see the material being sanded during the actual operation; he/she can only see material that is not covered by the blade. It is difficult to carry out a precise operation without repeatedly inspecting the work in progress and more closely reaching an approximation to the desired result. Hand-held tools cannot be re-applied precisely so that repeated inspection is not a good option for careful work.
It is a well known phenomenon that a disk having perforations becomes semi-transparent when spun at a moderate to high speed because of the persistence of image on the retina in the human eye--the "persistence of vision" effect. The image seen through a perforated spinning disk is further enhanced if there is a contrast in light and/or colour between the spinning disk and its background and/or foreground. To increase the width of the "window" or see-through viewing effect when a disk is spun, perforations are usually designed to overlay each other. There are many abrasive and rasping disks that make use of this phenomenon. Examples are those of F. Reidenback filed Aug. 31, 1953 U.S. Pat. No. 2,749,681 or J. C. Schwartz filed Mar. 26, 1985 U.S. Pat. No. 4,685,181.
Because of the presumed catastrophic consequences of protrusions into large apertures of perforated disks these inventions to date have relied on using many small perforations in the disk in relation to total disk size.